The mitotic (M) phase of the cell cycle alternates with the much longer interphase. The M phase includes mitosis and cytokinesis. Interphase accounts for 90% of the cell cycle. 1. The mitotic phase alternates with interphase in the cell cycle: an overview Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.4

During interphase the cell grows by producing proteins and cytoplasmic organelles, copies its chromosomes, and prepares for cell division. Interphase has three subphases: the G 1 phase (“first gap”) centered on growth, the S phase (“synthesis”) when the chromosomes are copied, the G 2 phase (“second gap”) where the cell completes preparations for cell division, and divides (M). The daughter cells may then repeat the cycle. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Mitosis is a continuum of changes. For description, mitosis is usually broken into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

By late interphase, the chromosomes have been duplicated but are loosely packed. The centrosomes have been duplicated and begin to organize microtubules into an aster (“star”). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.5a

In prophase, the chromosomes are tightly coiled, with sister chromatids joined together. The nucleoli disappear. The mitotic spindle begins to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.5b

During prometaphase, the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes. Microtubules from one pole attach to one of two kinetochores, special regions of the centromere, while microtubules from the other pole attach to the other kinetochore. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.5c

The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant between the poles, defining metaphase. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.5d

At anaphase, the centromeres divide, separating the sister chromatids. Each is now pulled toward the pole to which it is attached by spindle fibers. By the end, the two poles have equivalent collections of chromosomes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.5e

At telophase, the cell continues to elongate as free spindle fibers from each centrosome push off each other. Two nuclei begin for form, surrounded by the fragments of the parent’s nuclear envelope. Chromatin becomes less tightly coiled. Cytokinesis, division of the cytoplasm, begins. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.5f

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig left

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig right

Cytokinesis, division of the cytoplasm, typically follows mitosis. In animals, the first sign of cytokinesis (cleavage) is the appearance of a cleavage furrow in the cell surface near the old metaphase plate. 3. Cytokinesis divides the cytoplasm: a closer look Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.8a

On the cytoplasmic side of the cleavage furrow a contractile ring of actin microfilaments and the motor protein myosin form. Contraction of the ring pinches the cell in two. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.8a

Cytokinesis in plants, which have cell walls, involves a completely different mechanism. During telophase, vesicles from the Golgi coalesce at the metaphase plate, forming a cell plate. The plate enlarges until its membranes fuse with the plasma membrane at the perimeter, with the contents of the vesicles forming new wall material in between. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.8b

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 12.9

Prokaryotes reproduce by binary fission, not mitosis. Most bacterial genes are located on a single bacterial chromosome which consists of a circular DNA molecule and associated proteins. While bacteria do not have as many genes or DNA molecules as long as those in eukaryotes, their circular chromosome is still highly folded and coiled in the cell. 4. Mitosis in eukaryotes may have evolved from binary fission in bacteria Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In binary fission, chromosome replication begins at one point in the circular chromosome, the origin of replication site. These copied regions begin to move to opposite ends of the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

The mechanism behind the movement of the bacterial chromosome is still an open question. A previous hypothesis proposed that this movement was driven by the growth of new plasma membrane between the two origin regions. Recent observations have shown more directed movement, reminiscent of the poleward movement of eukaryotic chromosomes. However, mitotic spindles or even microtubules are unknown in bacteria. As the bacterial chromosome is replicating and the copied regions are moving to opposite ends of the cell, the bacterium continues to grow until it reaches twice its original size. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Cell division involves inward growth of the plasma membrane, dividing the parent cell into two daughter cells, each with a complete genome. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

It is quite a jump from binary fission to mitosis. Possible intermediate evolutionary steps are seen in the division of two types of unicellular algae. In dinoflagellates, replicated chromosomes are attached to the nuclear envelope. In diatoms, the spindle develops within the nucleus. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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